Images can be created by transmitting signals to the field of view and detecting the reflections from objects within the field or by detecting the attenuated signals after passing through the field of view. This may involve using an array of detectors to simultaneously obtain information about different regions in the field of view or scanning the field of view by sequentially receiving signals from different regions in the field of view.
Keating et al. U.S. Pat. No. 4,119,940, which is hereby incorporated by reference, discloses a sonar imaging system employing four transmitters sending omni-directional signals at different frequencies and an array of detectors to receive the signals. A processor is used to separate the different frequencies of the reflected signals, and it is mentioned that the use of plural frequencies increases the resolution.
Lee and Furgason, "Use of noise signals for multi-mode operation of phased arrays", J. Accoust. Soc. Am. 68(1), July 1980, pp. 320-328, discloses scanning using N different coded beams directed in different directions, a signal receiver, and processing to separate reflections having different codes; in the disclosed example two uncorrelated noise sources are used to produce beams in two directions, and two correlators are used to detect the reflected signals. Furgason, "Optimal Operation of Ultrasonic Correlation Systems", 1982 Ultrasonics Symposium, pp. 919-928, discloses using Golay codes, paired complementary binary codes, for the two coded beams transmitted in different directions. Lee and Furgason, "Golay Codes for Simultaneous Multi-Mode Operation in Phased Relays", 1982 Ultrasonics Symposium, pp. 821-825, similarly discloses using Golay codes and notes that they have good autocorrelation and zero cross-correlation.